1. Field of the Invention
This invention relates to an optical receiver circuit and an optical coupling device.
2. Background Art
An optical coupling device is used to enable signal transmission with electrical insulation between an input circuit and an output circuit. The optical coupling device includes an optical receiver circuit, which can receive an optical signal from a light-emitting element and convert the optical signal to an electrical signal.
Depending on the uses such as machine tools and process controllers, digital control using pulse signals is often applied, and hence it is necessary to reduce pulse width distortion.
Typically, to convert an optical signal to an electrical signal, a photocurrent detected by a PD (photodiode) is converted to a voltage signal by a transimpedance amplifier. The converted voltage signal is compared with an appropriate reference voltage by a comparator. To increase the accuracy of voltage comparison, the reference voltage is generated by superposing the output of another transimpedance amplifier having the same configuration with no photocurrent passed therethrough and an offset voltage as a bias.
In general, the efficiency of light emission from a light-emitting element decreases at high temperatures and across the ages, and there is also large variation (dispersion) between elements. Hence, typically, the reference voltage cannot be set to a high level in view of input decrease. On the other hand, the PD used as a light-receiving element has a high junction capacitance, which tends to blunt its waveform. Furthermore, the charge accumulated therein after extinction of input light is gradually released and increases the blunting of the falling edge. For these reasons, on the turn-off side, the transfer delay time is lengthened and tends to increase pulse width distortion.
Japanese Patent No. 3665635 discloses a technique related to an optical signal receiving circuit with reduced pulse width distortion, and an optical signal receiving semiconductor device. In this technique, a level shift circuit is provided between a transimpedance amplifier and a differential amplifier. The level shift circuit shifts the voltage to avoid the influence of saturation of the differential amplifier on the output signal.
However, in this technique, because the pulse is shifted entirely, the threshold changes between the first pulse and successive pulses, and the delay time varies accordingly. Furthermore, the delay time is adjusted with reference to the center value of the pulse. However, even if the optical signal from the light-emitting element is rapidly turned on/off, there is a certain delay in the response characteristics of the transimpedance amplifier. Therefore, such problems as increased delay time are caused when the pulse input becomes excessive.